1. Field of the Invention
The present invention relates to a method of precisely controlling a printhead carriage transportation velocity of a serial printer irrespective of environmental changes, aging, etc. which adversely affects a speed of a motor used for driving the carriage.
2. Description of Related Art
In order to realize a high quality printing in a serial printer, it is vital to precisely control a velocity of a printhead which is driven by a motor via a timing belt.
One known approach to realizing such requirements is to constantly detect a motor speed and check to see if a motor is driven at a predetermined reference speed. That is, the motor speed detected is compared with the reference (viz., desired) speed. Thereafter, a controller provided in a feedback loop controls the motor speed such as to reduce a difference between the predetermined reference motor speed and the motor speed actually detected. Thus, the motor speed is eventually converged to the predetermined speed.
By way of example, such a prior art is disclosed in Japanese Laid-open Patent Application No. 63-277487. In this prior art, if an actual motor speed is detected faster than a predetermined value, a current to be applied to the motor is reduced. On the contrary, if the motor speed is detected lower than the predetermined speed, the motor current is increased.
However, characteristics of a motor and associated mechanism for driving a printhead, are subjected to environmental changes such as ambient temperature changes and aging of printhead driving mechanism.
As is known, serial printing is implemented by moving linearly a carriage at a constant speed. Therefore, when such environmental changes occur, the carriage speed changes and thus, print position errors are inevitably induced.
In order to overcome such problems, according to another prior art disclosed by Japanese Laid-Open Patent Application No. 4-164669, the aforesaid aging and environmental changes are used as load changes. That is, the load changes are estimated. Thereafter, a motor driving current is calculated considering the load changes which have been obtained experimentally.
The latter mentioned prior art techniques undesirably require:
(1) Load changes should previously be estimated by conducting experiments; PA1 (2) It is required to determine time responses of a motor speed when a motor is initially energized; and PA1 (3) A memory is required which stores the experimental data.
Therefore, in order to precisely control the motor speed without incurring complicated hardware arrangment, it is highly desirable to eliminate the above mentioned problems inherent in the prior art.